Had some questions about rods. I don't understand if they're involved with night vision, then why are rods not stimulated in the dark but in the light they break down rhodopsin to retinal and opsin and then they become stimulated?
I thought they'd be stimulated in the dark if they're involved in night vision? :/
Cones need a whole lot of light to 'work'. By this I mean it takes quite a high light intensity to stimulate a generator potential that in turn causes an action potential to travel across the synaptic cleft to the one bipolar cell it's hooked up to.
However, rod cells need a lot less light to 'work', because there are three rod cells hooked up to one bipolar cell. This is called retinal convergence, and means that the potentials generated in the rod cells are summated and a low light intensity experienced by rod cells is much more likely to create an action potential in the bipolar cell than cone cells.
This means that in low light levels, each rod cell will give only a weak generator potential, but when these three potentials are added together across the synapse the result is strong enough to cause an action potential. They aren't stimulated by the dark, they just need less light to work! How would the rod cells get any information about our surroundings if they were just stimulated by the fact that it's dark?
The downside of this is that rod cells give an image with a lower resolution than cone cells - this is why everything looks quite grainy in the dark. They give a lower resolution because three rod cells are synapsed to one bipolar cell, so there's no way of distinguishing a signal between the three like there is with the one bipolar cell-to-one cone cell deal.
o--<>-o One cone cell synapsed to one bipolar cell
.O <- One bipolar cell
.^ <- One synapse
ooo <- 3 rod cells